This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application entitled Apparatus For Generating Dynamic Focus Signal earlier filed in the Korean Industrial Property Office on Aug. 14, 2001, and there duly assigned Serial No. 2001-0049035 by that Office.
1. Field of the Invention
The present invention relates to an apparatus for generating a dynamic focus signal in a display apparatus using a Cathode Display Tube (CDT) or, and more particularly, to an apparatus for generating a dynamic focus signal appropriate for a flat-type CDT, or flat-screen Cathode Ray Tube (CRT).
2. Description of the Related Art
Since distances from the electron gun to each area of the screen are different in a display apparatus using a CDT or CRT, dynamic focus for adjusting the focus of an electron beam with respect to an area of the screen is needed so that an electron beam generated in the electron gun is landed with a correct focus.
That is, since the distance between the electron gun and the center of the screen is short, an electron beam directed to the center of the screen is needed to correspond to a short focal length. However, the distance between the electron gun and the right or left area of the screen is longer than that between the electron gun and the center of the screen, an electron beam directed to the right or left area of the screen is needed to correspond to a longer focal length. As this, adjusting the focal length of an electron beam with respect to an area of the screen is referred to as dynamic focus.
The focus of an electron beam is controlled by a focus signal applied to the focus electrode of the electron gun.
FIG. 1 is a waveform diagram of exemplary focus signals. The focus signals of FIG. 1 show the waveforms of horizontal focus signals and a vertical focus signal in one vertical cycle. Referring to FIG. 1, each horizontal focus signal has high levels at the beginning and end of a horizontal scanning period, and has a low level in the middle area of the horizontal scanning period. According to this, in one horizontal cycle, the focus voltages of the left and right areas in the screen are different from the focus voltage of the center of the screen.
Meanwhile, in the waveforms of FIG. 1, in one vertical cycle, the waveform of each horizontal focus signal is uniform. This means that horizontal dynamic focus controls for the center and corners of the screen are done in a uniform way.
Though with the focus signals shown in FIG. 1, correct focus may be achieved to a degree in an ordinary non-flat-screen CRT, precise focus cannot be achieved in a flat-screen CRT which is used now or will be used in the future.
The reason is because the distance between the electron gun and the corners of the screen (the top left-hand corner, top right-hand corner, bottom left-hand corner, and bottom right-hand corner) of the flat-screen CRT is longer than that of the conventional non-flat-screen CRT s. Therefore, with the dynamic focus signal having the same amplitude (V2) of the horizontal focus signal for each corner of the screen as the amplitude (V1) of the horizontal focus signal for the center of the screen, as shown in FIG. 1, it is difficult to adjust focuses for corner areas of the screen.
To solve the above problems, it is an objective of the present invention to provide an apparatus for generating a dynamic focus waveform appropriate for a flat-type CDT, or flat-screen CRT.
To accomplish the objective of the present invention, there is provided an apparatus for generating a dynamic focus signal appropriate for a flat-screen CRT, the apparatus having a first multiplier for receiving a horizontal sawtooth wave and outputting the square component of the wave; a second multiplier for receiving a vertical sawtooth wave and outputting the square component of the wave; a third multiplier for multiplying the outputs of the first and the second multipliers; and a mixer for mixing the outputs of the first through the third multipliers and outputting the result as a dynamic focus signal.